A process for the ammoxidation of 3,4-difluorotoluene to 3,4-difluorobenzonitrile

Abstract

The application discloses a method for preparing 3,4-difluorobenzonitrile by 3,4-difluorotoluene ammoxidation, which is prepared from 3,4-difluorotoluene, ammonia and oxygen-containing gas through four steps of raw material vaporization mixing, gas phase catalytic ammoxidation, quenching and condensation separation and crude product refining. 1.0 Cr a BPO x The carrier is silicon dioxide or alumina; the reaction temperature is 320-420 DEG C; the space velocity is 500-5000 h ‑ The molar ratio of 3,4-difluorotoluene, NH3 and O2 is 1:5:1; the method is one-step gas phase catalysis instead of traditional multi-step pot-type reaction, no chlorine, antimony pentachloride and large amount of high-boiling-point solvent are used, chlorine-containing waste is significantly reduced, the process is simple, and the degree of continuity is high; the one-way yield of the product is greater than or equal to 92%, the purity is greater than or equal to 99.5%, the catalyst stability is good, the post-treatment is simple, the energy consumption and environmental protection cost are greatly reduced, and the method is suitable for industrial large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of chemical production technology, specifically relating to a method for the ammoxidation of 3,4-difluorotoluene to produce 3,4-difluorobenzonitrile. Background Technology

[0002] 3,4-Difluorobenzonitrile is an important intermediate in fluorinated aromatic fine chemicals. Due to the presence of fluorine and cyano groups in its molecular structure, it possesses excellent stability, lipophilicity, and reactivity, making it widely used in pharmaceuticals, pesticides, novel fluorinated liquid crystal materials, and electronic chemicals. In the pesticide field, 3,4-difluorobenzonitrile is a key intermediate in the synthesis of highly effective, low-toxicity, and low-residue herbicides such as cyhalofop-butyl and flufenacet. These herbicides exhibit excellent control of barnyard grass and Echinochloa crus-galli in paddy fields, with high crop safety, and are currently the mainstream products in the global paddy field weed control sector. In the pharmaceutical field, 3,4-difluorobenzonitrile can be used to synthesize intermediates for antifungal, antiviral, and central nervous system drugs. In the liquid crystal materials field, fluorinated benzonitrile compounds possess advantages such as high voltage retention, low viscosity, and a wide nematic phase temperature range, making them important components of high-end TFT-LCD liquid crystal materials. With the rapid development of downstream pharmaceutical, pesticide, and electronic information industries, the market demand for 3,4-difluorobenzonitrile continues to grow, placing higher demands on its synthesis process in terms of yield, purity, cost, and environmental friendliness.

[0003] Currently, the industrial synthesis route of 3,4-difluorobenzonitrile mainly uses aromatic compounds as raw materials and introduces cyano groups through functional group transformation. Existing mainstream processes use 3,4-difluorotoluene as a starting material, preparing the target product through a multi-step batch reaction. Among them, patent technology with publication number CN109180524A discloses a method for preparing 3,4-difluorobenzonitrile from 3,4-difluorotoluene via chlorination, hydrolysis, oxime formation, and dehydration. The specific steps are as follows: 3,4-difluorotoluene is mixed with antimony pentachloride, and chlorine gas is introduced under light to carry out a side-chain chlorination reaction, yielding a polychlorinated benzyl intermediate; the chlorinated product is hydrolyzed in a polar aprotic solvent with zinc chloride catalysis to generate 3,4-difluorobenzaldehyde; benzaldehyde undergoes an oxime reaction with hydroxylamine hydrochloride in an acidic solvent, followed by staged heating and dehydration to obtain crude 3,4-difluorobenzonitrile; the crude product is recrystallized, decolorized, and dried to obtain a refined product. This process has achieved the synthesis of 3,4-difluorobenzonitrile to a certain extent, but it has insurmountable technical defects in industrial production, as follows:

[0004] First, the reaction steps are lengthy and the process flow is complex, limiting the overall yield. The process comprises five independent unit operations: chlorination, hydrolysis, oxime, dehydration, and purification. Each step involves material transfer, intermediate product separation, and purification, resulting in significant process losses and limited potential for improving the overall yield. The multi-step reaction significantly extends the production cycle, with single-batch production exceeding 15 hours, leading to low equipment utilization and making it difficult to meet the demands of large-scale continuous production.

[0005] Secondly, the extensive use of organic solvents and chlorine-containing reagents places immense environmental pressure on the process. The process requires the use of high-boiling-point polar aprotic solvents such as N-methylpyrrolidone, γ-butyrolactone, dimethyl sulfoxide, and N,N-dimethylformamide. These solvents have high boiling points and low volatility, requiring high-temperature, vacuum distillation for recovery, resulting in extremely high energy consumption and difficulty in complete recovery. The single-use loss rate exceeds 8%, leading to persistently high production costs. Simultaneously, the reaction requires chlorine-containing reagents such as chlorine gas, antimony pentachloride, and zinc chloride. The chlorination reaction produces large amounts of hydrogen chloride gas, and the hydrolysis and oxime processes generate high-concentration chlorine-containing wastewater. This wastewater has a high chloride ion content and is difficult to biodegrade, requiring complex pretreatment before it can meet discharge standards. The cost of treating these three wastes accounts for more than 25% of the total production cost.

[0006] Third, numerous side reactions occur, making product purification difficult and limiting product purity. The photo-induced side-chain chlorination reaction exhibits poor selectivity, easily generating mixtures of benzyl chloride, dichlorobenzyl chloride, and trichlorobenzyl chloride. Strict control of chlorination rate and reaction time is necessary, but byproduct formation remains difficult to avoid. During oxime dehydration, benzaldehyde condensation and hydroxylamine decomposition easily occur under acidic conditions, producing tar-like impurities and resulting in high product color. Multiple recrystallization and decolorization processes are required to achieve the required purity, further reducing the overall yield and increasing purification costs.

[0007] Fourth, the process suffers from low levels of automation and continuity, resulting in high safety risks. Primarily employing batch reactor reactions, this process involves hazardous operations such as chlorine gas introduction, high-temperature reflux, and negative pressure solvent removal. The numerous manual intervention steps increase the risk of accidents due to operational errors, leading to uncontrolled reactions and material leaks. Furthermore, the batch operation results in poor product quality stability, with significant fluctuations in purity and yield between different batches, making it difficult to meet the stringent quality requirements of high-end pharmaceuticals and liquid crystal materials for intermediates.

[0008] In summary, existing multi-step batch synthesis processes suffer from numerous problems, including excessive steps, high solvent consumption, significant amounts of chlorine-containing waste, complex purification processes, and low levels of continuity, severely hindering the green and efficient development of the 3,4-difluorobenzonitrile industry. Therefore, the development of a novel synthesis process that is simple in steps, free of chlorine-containing reagents, has low solvent consumption, high yield, and high continuity to replace traditional multi-step reaction routes has become a pressing technical problem for those skilled in the art. Summary of the Invention

[0009] To address the aforementioned shortcomings of existing technologies, the present invention aims to provide a method for the ammoxidation of 3,4-difluorotoluene to 3,4-difluorobenzonitrile, thereby solving the technical problems of traditional processes, such as numerous reaction steps, large amounts of chlorine-containing waste, complex solvent recovery, high purification difficulty, and low degree of continuous operation. The present invention employs a gas-phase ammoxidation method, using 3,4-difluorotoluene, ammonia, and air as raw materials, directly converting them to 3,4-difluorobenzonitrile under the action of a vanadium-chromium-boron-phosphorus composite oxide catalyst. This method offers advantages such as a short process flow, high yield, high purity, minimal waste, and high degree of continuous operation, making it suitable for large-scale industrial production.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows: A method for the ammoxidation of 3,4-difluorotoluene to 3,4-difluorobenzonitrile includes the following steps: Step 1, Raw material pretreatment and mixed gas preparation: 3,4-difluorotoluene raw material is subjected to vacuum vaporization and superheating treatment, and then fed into a static mixer with ammonia and preheated oxygen-containing gas in a set ratio to obtain a uniform reaction mixed gas. Step 2, gas-phase catalytic ammonia oxidation reaction: The reaction mixture obtained in Step 1 is passed into a fixed bed or fluidized bed reactor packed with ammonia oxidation catalyst, and the gas-phase catalytic ammonia oxidation reaction is carried out under set temperature, pressure and space velocity to obtain reaction product gas containing 3,4-difluorobenzonitrile. Step 3, rapid cooling and gas-liquid separation of reaction products: The reaction product gas obtained in step 2 is subjected to high-temperature rapid cooling and low-temperature condensation treatment in sequence to separate crude 3,4-difluorobenzonitrile liquid and non-condensable gas. The non-condensable gas is discharged after alkaline washing and incineration to meet the standards. Step 4, Crude product refining and refined product preparation: The crude 3,4-difluorobenzonitrile liquid obtained in Step 3 is subjected to vacuum distillation and vacuum drying in sequence to finally obtain high-purity 3,4-difluorobenzonitrile refined product.

[0011] In a preferred embodiment of the present invention, the raw material molar ratio in step 1 is 3,4-difluorotoluene:NH3:O2=1:5:1, the oxygen-containing gas is air, oxygen-enriched air or pure oxygen, the air is preheated to 150-250°C, and used after dehydration and dust removal.

[0012] In a preferred embodiment of the present invention, in step 1, the vaporization temperature of 3,4-difluorotoluene is 120-180°C, the superheating temperature is 180-250°C, and the residence time of the mixed gas in the static mixer is 0.5-3 seconds to ensure uniform mixing without localized excessive concentration.

[0013] In a preferred embodiment of the present invention, in step 2, the ammonia oxidation catalyst is a supported composite oxide catalyst, and the general formula of the active component is expressed in atomic ratio as follows: V 1.0 Cr a B P O x Where a = 0.3–1.5, β = 0.05–0.5, γ = 0.1–1.0, and x is the number of oxygen atoms required to satisfy valence equilibrium; the support is selected from one or more of SiO2, Al2O3, ZrO2, and TiO2, and the support accounts for 30%–70% of the total mass of the catalyst; the ammonia oxidation reaction temperature is 320–420℃, the pressure is 0.02 MPa–0.1 MPa (gauge pressure), and the space velocity is 500–5000 h⁻¹. -1 The raw material molar ratio is 3,4-difluorotoluene:NH3:O2 = 1:5:1.

[0014] In a preferred embodiment of the present invention, the supported composite oxide catalyst particles are spherical, strip-shaped, or microsphere-shaped with a particle size of 20-100 mesh. The SiO2 support is one of fumed silica, precipitated silica, or silica gel, and the SiO2 specific surface area is 30-120 m² / g. 2 / g, pore volume 0.3-1.0cm 3 / g.

[0015] In a preferred embodiment of the present invention, in step 2, the ammonia oxidation reaction temperature is preferably 350-390°C, and the reaction pressure is 0.05MPa-0.2MPa; the reactor adopts segmented temperature control, with the inlet temperature being 10-30°C lower than the main reaction section temperature, and the outlet temperature being 5-15°C lower than the main reaction section temperature.

[0016] In a preferred embodiment of the present invention, the reactor bed is equipped with multiple temperature measurement points. When the hot spot temperature exceeds 400°C, the hot spot temperature is controlled below 390°C by reducing the feed load, increasing the inert dilution ratio, or increasing the heat exchange flow rate.

[0017] In a preferred embodiment of the present invention, in step 3, the high-temperature quenching temperature is 100-160°C, and circulating heat transfer oil is used for quenching; the low-temperature condensation temperature is 30-50°C, and circulating cooling water is used for condensation; the non-condensable gas is sent to the incinerator for treatment after being washed with 10%-20% NaOH alkali.

[0018] In a preferred embodiment of the present invention, in step 4, the vacuum degree of the reduced pressure distillation is -0.095MPa to -0.08MPa, and the temperature of the distillation vessel is ≤180℃.

[0019] In a preferred embodiment of the present invention, the ammonia oxidation catalyst is prepared by sol-gel method or equal volume impregnation method. Vanadium source, chromium source, boron source and phosphorus source are dissolved in water-oxalic acid mixture, added to support and stirred for 1-4 h, dried at 110-130℃ for 8-16 h, and then calcined in stages: 200℃ / 1 h → 400℃ / 2 h → 500-600℃ / 3-6 h. Before use, the catalyst is activated in ammonia-air mixture at 350-400℃ for 2-5 h.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. Significantly simplified process flow: The multi-step reaction of chlorination, hydrolysis, oxime, and dehydration is reduced to a single gas-phase catalytic reaction, shortening the production cycle by more than 70%, significantly reducing equipment investment and operating costs, and greatly improving the degree of continuity and automation.

[0021] 2. Green and environmentally friendly with minimal waste: The entire process does not use chlorine-containing reagents such as chlorine, antimony pentachloride, and zinc chloride, and there is no high-concentration chlorine-containing wastewater or hazardous waste generated. No high-boiling-point solvents are required, solvent consumption is reduced by 90%, and environmental protection costs are significantly reduced.

[0022] 3. High catalytic activity and good selectivity: The use of vanadium-chromium-boron-phosphorus composite oxide catalyst can effectively suppress fluorine shedding and excessive oxidation side reactions, with a single-pass yield of ≥92% and product purity of ≥99.5%, ensuring stable quality that meets the requirements of pharmaceuticals and high-end liquid crystal materials.

[0023] 4. Mild and easy-to-control reaction conditions: The reaction temperature is 320-420℃, and the operation is from atmospheric pressure to low pressure. The bed hot spot is easy to control, the catalyst has a long lifespan and can operate stably for a long time, with low safety risks.

[0024] 5. High atom economy and low cost: The byproducts are only water and a small amount of carbon dioxide. The raw material utilization rate is high, the post-processing is simple, and the overall product cost is significantly lower than that of traditional processes, giving it a strong industrial competitiveness. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments or prior art, the drawings used in the description of the embodiments or prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 The flowchart illustrates a method for the ammoxidation of 3,4-difluorotoluene to produce 3,4-difluorobenzonitrile, as provided in an embodiment of the present invention.

[0027] Figure 2 This is a schematic diagram of the connection of some equipment in a method for producing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to an embodiment of the present invention.

[0028] Figure 3 The XRD pattern of a vanadium-chromium-boron-phosphorus composite oxide catalyst provided in an embodiment of the present invention.

[0029] Figure 4This invention provides a curve showing the effect of reaction temperature on the conversion rate of 3,4-difluorotoluene and the selectivity of 3,4-difluorobenzonitrile. Detailed Implementation

[0030] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0031] like Figure 1 and Figure 2 As shown, this embodiment of the invention provides a method for the ammoxidation of 3,4-difluorotoluene to produce 3,4-difluorobenzonitrile, comprising the following steps: Step 1, Raw material pretreatment and mixed gas preparation: 3,4-difluorotoluene raw material is subjected to vacuum vaporization and superheating treatment, and then fed into a static mixer with ammonia and preheated oxygen-containing gas in a set ratio to obtain a uniform reaction mixed gas.

[0032] Specifically, the 3,4-difluorotoluene raw material is subjected to vacuum vaporization and superheating treatment, and then fed into a static mixer with ammonia and preheated oxygen-containing gas in a set ratio to obtain a uniform reaction mixture.

[0033] Step 2, gas-phase catalytic ammonia oxidation reaction: The reaction mixture obtained in Step 1 is passed into a fixed bed or fluidized bed reactor packed with ammonia oxidation catalyst, and the gas-phase catalytic ammonia oxidation reaction is carried out under set temperature, pressure and space velocity to obtain reaction product gas containing 3,4-difluorobenzonitrile.

[0034] Specifically, the reaction mixture obtained in step 1 is introduced into a fixed bed or fluidized bed reactor packed with an ammonia oxidation catalyst. Under set temperature, pressure and space velocity, a gas-phase catalytic ammonia oxidation reaction is carried out. Under the action of the catalyst active sites, methyl groups react with ammonia and oxygen to undergo oxidative dehydrogenation, coupling and dehydration reactions, directly generating a reaction product gas containing 3,4-difluorobenzonitrile.

[0035] Step 3, rapid cooling and gas-liquid separation of reaction products: The reaction product gas obtained in step 2 is subjected to high-temperature rapid cooling and low-temperature condensation treatment in sequence to separate crude 3,4-difluorobenzonitrile liquid and non-condensable gas. The non-condensable gas is discharged after alkaline washing and incineration to meet the standards.

[0036] Specifically, the high-temperature reaction product gas obtained in step 2 is first sent to a quench cooler to suppress the occurrence of side reactions through high-temperature quenching, and then enters a condenser for low-temperature condensation to achieve gas-liquid separation, obtaining crude 3,4-difluorobenzonitrile liquid and non-condensable gas. The non-condensable gas is treated by alkaline washing and incineration before being discharged in compliance with standards.

[0037] Step 4, Crude product refining and refined product preparation: The crude 3,4-difluorobenzonitrile liquid obtained in Step 3 is subjected to vacuum distillation and vacuum drying in sequence to finally obtain high-purity 3,4-difluorobenzonitrile refined product.

[0038] Specifically, the crude 3,4-difluorobenzonitrile liquid obtained in step 3 is fed into a distillation column, where light and heavy components are removed by vacuum distillation, and then dried under vacuum to finally obtain high-purity 3,4-difluorobenzonitrile.

[0039] In step 1, the raw material molar ratio is 3,4-difluorotoluene:NH3:O2=1:5:1; the oxygen-containing gas is air, oxygen-enriched air or pure oxygen, and the air is preheated to 150-250℃ and used after dehydration and dust removal.

[0040] In step 1, the vaporization temperature of 3,4-difluorotoluene is 120-180℃, the superheating temperature is 180-250℃, and the residence time of the mixed gas in the static mixer is 0.5-3s to ensure uniform mixing without excessively high local concentrations.

[0041] In step 2, the ammonia oxidation catalyst is a supported composite oxide catalyst, and the general formula of the active component is as follows (in atomic ratio): V 1.0 Cr a B P O x Where a = 0.3–1.5, β = 0.05–0.5, γ = 0.1–1.0, and x is the number of oxygen atoms required to satisfy valence equilibrium; the support is selected from one or more of SiO2, Al2O3, ZrO2, and TiO2, and the support accounts for 30%–70% of the total mass of the catalyst; the ammonia oxidation reaction temperature is 320–420℃, the pressure is 0.02 MPa–0.1 MPa (gauge pressure), and the space velocity is 500–5000 h⁻¹. -1 The raw material molar ratio of 3,4-difluorotoluene:NH3:O2 = 1:5:1 is referenced. Figure 4 .

[0042] The supported composite oxide catalyst particles are spherical, strip-shaped, or microsphere-shaped, with a particle size of 20–100 mesh. The SiO2 support is one of fumed silica, precipitated silica, or silica gel, and the SiO2 specific surface area is 30–120 m² / g. 2 / g, pore volume 0.3-1.0cm 3 / g.

[0043] In step 2, the preferred temperature for the ammonia oxidation reaction is 350–390°C, and the reaction pressure is 0.05 MPa–0.2 MPa. The reactor adopts segmented temperature control, with the inlet temperature being 10–30°C lower than the main reaction temperature and the outlet temperature being 5–15°C lower than the main reaction temperature.

[0044] The reactor bed is equipped with multiple temperature measurement points. When the hot spot temperature exceeds 400℃, the hot spot temperature is controlled below 390℃ by reducing the feed load, increasing the inert dilution ratio, or increasing the heat exchange flow rate.

[0045] In step 3, the high-temperature quenching temperature is 100-160℃, and circulating heat transfer oil is used for quenching; the low-temperature condensation temperature is 30-50℃, and circulating cooling water is used for condensation; the non-condensable gas is sent to the incinerator for treatment after being washed with 10%-20% NaOH alkali.

[0046] In step 4, the vacuum degree of the reduced pressure distillation is -0.095MPa to -0.08MPa, and the temperature of the distillation vessel is ≤180℃.

[0047] The ammonia oxidation catalyst is prepared by sol-gel method or equal volume impregnation method. Vanadium source, chromium source, boron source and phosphorus source are dissolved in water-oxalic acid mixture, added to support and stirred for 1-4 h, dried at 110-130℃ for 8-16 h, and then calcined in stages: 200℃ / 1 h → 400℃ / 2 h → 500-600℃ / 3-6 h. Before use, the catalyst is activated in ammonia-air mixture at 350-400℃ for 2-5 h.

[0048] Specifically, in this embodiment, ammonium metavanadate, chromium nitrate, boric acid, and phosphoric acid are added to a water-oxalic acid mixed solution in an atomic ratio of V:Cr:B:P = 1.0:0.8:0.2:0.5. The solution is heated to 80°C and stirred until completely dissolved. A precipitated SiO2 support (with a specific surface area of ​​80 m²) is then added. 2 / g, pore volume 0.6cm 3 (g), continue stirring for 2 hours to obtain a uniform slurry; the slurry is dried at 120℃ for 12 hours, and then calcined in stages: 200℃ / 1h → 400℃ / 2h → 550℃ / 4h, to obtain a 40-60 mesh catalyst with an active component loading of 45%, denoted as catalyst A. Before use, the catalyst is activated in an ammonia-air mixture at 350-400℃ for 2-5 hours to improve its catalytic activity. (Refer to...) Figure 3 .

[0049] The reaction mechanism of this invention is based on the Mars-van Krevelen redox mechanism: the methyl group on the side chain of 3,4-difluorotoluene is reacted with catalyst V. 5+ / V 4+Redox active sites adsorb and dehydrogenate to generate benzyl radicals; ammonia activates at the acidic sites of the catalyst to generate NHx active intermediates, which couple with benzyl radicals to form imine intermediates; the imine intermediates undergo dehydrogenation and dehydration under the action of lattice oxygen to generate 3,4-difluorobenzonitrile; gaseous oxygen replenishes the lattice oxygen of the catalyst, completing the catalytic cycle. Cr in the catalyst regulates the redox capacity, while B and P elements adjust surface acidity and oxygen migration rate, inhibiting side reactions such as fluorine atom shedding, benzene ring opening, and over-oxidation, significantly improving reaction selectivity and yield.

[0050] Example 1 Step 1, Raw material pretreatment and mixed gas preparation: 3,4-difluorotoluene is fed into a vaporizer, vaporized at 150°C and superheated at 200°C, and mixed with ammonia and air preheated to 200°C. The molar ratio of 3,4-difluorotoluene:NH3:O2 is controlled to be 1:2.0:4.2. The mixture is then mixed in a static mixer for 1 second to obtain a reaction mixed gas.

[0051] Step 2, gas-phase catalytic ammonia oxidation reaction: The mixed gas is introduced into a fixed-bed reactor packed with catalyst A. The reaction temperature is 370℃, the pressure is 0.1MPa (gauge pressure), and the space velocity is 1200h⁻¹. - ¹, to carry out gas-phase ammonia oxidation reaction.

[0052] Step 3, rapid cooling and gas-liquid separation of reaction products: The reaction product gas is rapidly cooled by 130°C heat transfer oil and condensed by 40°C cooling water. The gas-liquid separation yields crude 3,4-difluorobenzonitrile liquid. The non-condensable gas is washed with 15% NaOH alkali and incinerated before being discharged.

[0053] Step 4, Crude product refining and refined product preparation: The crude product was subjected to vacuum distillation at -0.09 MPa and a kettle temperature of 170℃. The distillation product was recrystallized at 10℃ using toluene as solvent, decolorized with 2% activated carbon, and vacuum dried at 60℃ for 3 hours to obtain refined 3,4-difluorobenzonitrile.

[0054] The product has a single-pass yield of 93.2%, a purity of 99.62%, and a color of ≤10 APHA, according to the test results.

[0055] Example 2 The catalyst atomic ratio V:Cr:B:P = 1.0:1.0:0.15:0.6, and the remaining preparation conditions are the same as in the catalyst preparation example, denoted as catalyst B. The reaction temperature is 380℃, the pressure is 0.12 MPa, the molar ratio is 1:2.2:4.5, and the space velocity is 1500 h⁻¹. -1 The remaining steps are the same as in Example 1.

[0056] The product has a single-pass yield of 94.1%, a purity of 99.71%, and a color of ≤8 APHA.

[0057] Example 3 The catalyst atomic ratio V:Cr:B:P = 1.0:0.6:0.25:0.4, and the remaining preparation conditions are the same as in the catalyst preparation example, denoted as catalyst C. The reaction temperature is 365℃, the pressure is 0.08 MPa, the molar ratio is 1:1.8:4.0, and the space velocity is 1000 h⁻¹. -1 The remaining steps are the same as in Example 1.

[0058] The product has a single-pass yield of 92.7%, a purity of 99.53%, and a color of ≤10 APHA.

[0059] Example 4 A fluidized bed reactor was used, loaded with catalyst B, at a reaction temperature of 375℃, a pressure of 0.1MPa, and a space velocity of 2000h⁻¹. -1 The remaining steps are the same as in Example 1.

[0060] The product has a single-pass yield of 93.5%, a purity of 99.65%, and operates stably in the fluidized bed with no catalyst agglomeration.

[0061] Example 5 Catalyst B was prepared by diluting and filling the hot spot area with 20% quartz sand, with a reaction temperature of 380℃ and a pressure of 0.1MPa. The remaining steps were the same as in Example 1.

[0062] The product has a single-pass yield of 94.3%, a purity of 99.80%, and the catalyst exhibits an activity decrease of ≤1.5% after 1000 hours of continuous operation, demonstrating excellent stability.

[0063] Comparative Example 1 3,4-Difluorobenzonitrile was prepared using the chlorination-hydrolysis-oxime-dehydration process disclosed in CN109180524A.

[0064] The product yield is 88.2%, the purity is 98.45%, the amount of chlorine-containing wastewater generated is 12.5t / t of product, and the energy consumption for solvent recovery is high.

[0065] Comparative Example 2 The catalyst does not contain B or P elements; the active component is V. 1.0 Cr 0.8 O x The remaining conditions are the same as in Example 1.

[0066] The product had a single-pass yield of 82.3% and a selectivity of 85.6%. However, the product had increased byproducts of fluorine shedding and excessive oxidation, resulting in a purity of only 96.21%.

[0067] Comparative Example 3 The reaction temperature was raised to 430°C, and the other conditions were the same as in Example 1.

[0068] Excessive bed hot spot temperature exacerbated the side reactions of benzene ring opening and excessive oxidation, resulting in a product yield of 85.7% and a purity of 97.32%.

[0069] Test case The product yield, purity, waste generation, and production cycle of Examples 1-5 were compared with those of Comparative Examples 1-3. The results are shown in the table below: sheet

[0070] As shown in the table above, the product yield and purity of the embodiments of the present invention are much higher than those of the comparative example, no chlorine-containing wastewater is generated, the production cycle is greatly shortened, the solvent consumption is extremely low, and the overall performance is significantly better than that of the traditional process.

[0071] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for the ammoxidation of 3,4-difluorotoluene to 3,4-difluorobenzonitrile, characterized in that, Includes the following steps: Step 1, Raw material pretreatment and mixed gas preparation: 3,4-difluorotoluene raw material is subjected to vacuum vaporization and superheating treatment, and then fed into a static mixer with ammonia and preheated oxygen-containing gas in a set ratio to obtain a uniform reaction mixed gas. Step 2, gas-phase catalytic ammonia oxidation reaction: The reaction mixture obtained in Step 1 is passed into a fixed bed or fluidized bed reactor packed with ammonia oxidation catalyst, and the gas-phase catalytic ammonia oxidation reaction is carried out under set temperature, pressure and space velocity to obtain reaction product gas containing 3,4-difluorobenzonitrile. Step 3, rapid cooling and gas-liquid separation of reaction products: The reaction product gas obtained in step 2 is subjected to high-temperature rapid cooling and low-temperature condensation treatment in sequence to separate crude 3,4-difluorobenzonitrile liquid and non-condensable gas. The non-condensable gas is discharged after alkaline washing and incineration to meet the standards. Step 4, Crude product refining and refined product preparation: The crude 3,4-difluorobenzonitrile liquid obtained in Step 3 is subjected to vacuum distillation and vacuum drying in sequence to finally obtain high-purity 3,4-difluorobenzonitrile refined product.

2. The method for preparing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to claim 1, characterized in that, In step 1, the raw material molar ratio is 3,4-difluorotoluene:NH3:O2=1:5:

1. The oxygen-containing gas is air, oxygen-enriched air, or pure oxygen. The air is preheated to 150-250℃ and used after dehydration and dust removal.

3. The method for preparing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to claim 2, characterized in that, In step 1, the vaporization temperature of 3,4-difluorotoluene is 120-180℃, the superheating temperature is 180-250℃, and the residence time of the mixed gas in the static mixer is 0.5-3s to ensure uniform mixing without excessively high local concentrations.

4. The method for preparing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to claim 1, characterized in that, In step 2, the ammonia oxidation catalyst is a supported composite oxide catalyst, and the general formula of the active component is as follows (in atomic ratio): V 1.0 Cr a B P O x Where a = 0.3–1.5, β = 0.05–0.5, γ = 0.1–1.0, and x is the number of oxygen atoms required to satisfy valence equilibrium; the support is selected from one or more of SiO2, Al2O3, ZrO2, and TiO2, and the support accounts for 30%–70% of the total mass of the catalyst; the ammonia oxidation reaction temperature is 320–420℃, the pressure is 0.02 MPa–0.1 MPa (gauge pressure), and the space velocity is 500–5000 h⁻¹. -1 The raw material molar ratio is 3,4-difluorotoluene:NH3:O2 = 1:5:

1.

5. The method for preparing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to claim 4, characterized in that, The supported composite oxide catalyst particles are spherical, strip-shaped, or microsphere-shaped, with a particle size of 20–100 mesh. The SiO2 support is one of fumed silica, precipitated silica, or silica gel, and the SiO2 specific surface area is 30–120 m² / g. 2 / g, pore volume 0.3-1.0cm 3 / g.

6. The method for preparing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to claim 1, characterized in that, In step 2, the preferred temperature for the ammonia oxidation reaction is 350–390°C, and the reaction pressure is 0.05 MPa–0.2 MPa. The reactor adopts segmented temperature control, with the inlet temperature being 10–30°C lower than the main reaction temperature and the outlet temperature being 5–15°C lower than the main reaction temperature.

7. The method for preparing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to claim 6, characterized in that, The reactor bed is equipped with multiple temperature measurement points. When the hot spot temperature exceeds 400℃, the hot spot temperature is controlled below 390℃ by reducing the feed load, increasing the inert dilution ratio, or increasing the heat exchange flow rate.

8. The method for preparing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to claim 1, characterized in that, In step 3, the high-temperature quenching temperature is 100-160℃, and circulating heat transfer oil is used for quenching; the low-temperature condensation temperature is 30-50℃, and circulating cooling water is used for condensation; the non-condensable gas is sent to the incinerator for treatment after being washed with 10%-20% NaOH alkali.

9. The method for preparing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to claim 1, characterized in that, In step 4, the vacuum degree of the reduced pressure distillation is -0.095MPa to -0.08MPa, and the temperature of the distillation vessel is ≤180℃.

10. The method for preparing 3,4-difluorobenzonitrile by ammoxidation of 3,4-difluorotoluene according to claim 1, characterized in that, The ammonia oxidation catalyst is prepared by sol-gel method or equal volume impregnation method. Vanadium source, chromium source, boron source and phosphorus source are dissolved in water-oxalic acid mixture, added to support and stirred for 1-4 h, dried at 110-130℃ for 8-16 h, and then calcined in stages: 200℃ / 1 h → 400℃ / 2 h → 500-600℃ / 3-6 h. Before use, the catalyst is activated in ammonia-air mixture at 350-400℃ for 2-5 h.

Images